1. Field of the Invention
The present invention relates to optical apparatus, and more particularly, to an optical apparatus for amplifying an optical signal.
2. Description of the Related Art
In recent years, much attention and development are directed to DWDM (Dense Wavelength Division Multiplexing) which permits a single optical fiber to accommodate several tens to several hundreds of wavelength channels each capable of transmitting data at high rates of 600 Mbps to 10 Gbps, thereby enabling data transmission of an extremely large total capacity of the order of Tbps.
In optical amplification techniques for WDM systems including DWDM, EDFA (Erbium-Doped Fiber Amplifier) is widely used. EDFA is an optical amplifier using an EDF (Erbium (Er3+)-Doped Fiber) as an amplification medium. An optical signal is propagated through the EDF with pump light introduced to the EDF, so that the optical signal is amplified by the stimulated emission then induced. The EDFA has a wide gain bandwidth and is capable of collectively amplifying multiple optical signals in the wavelength band; therefore, the EDFA is used as a primary device in WDM repeaters.
FIG. 12 shows the configuration of a conventional EDFA. An optical amplifier (EDFA) 100 comprises a coupler C1, a pump LD (Laser Diode) 101, an EDF 102, optical isolators 103 and 107, a gain equalizer 104, a BS (Beam Splitter) 105, a variable attenuator (ATT) 106, a PD (Photo-Diode) 108, and a controller 109.
An input optical signal is introduced, together with pump light emitted from the pump LD 101, into the EDF 102 via the coupler C1, whereby the optical signal is amplified. The amplified optical signal is passed through the optical isolator 103 and the gain thereof is equalized (gain-wavelength characteristic of the EDF 102 is flattened) by the gain equalizer 104.
After passing through the gain equalizer 104, the optical signal is split into two by the BS 105. The variable attenuator 106 controls the level of one split optical signal, and the level-controlled optical signal is output through the optical isolator 107. The PD 108 converts the other split optical signal to an electrical signal, which is monitored by the controller 109. In accordance with the monitoring result, the controller 109 controls the power of pump light emitted from the pump LD 101.
Each of the optical isolators permits only the required light to transmit therethrough in the direction indicated by the arrow in the figure and does not transmit light in the opposite direction. Accordingly, by arranging the optical isolator 107 in the illustrated position, it is possible to cut off feedback light of the optical signal returned from a reflecting point in the optical fiber transmission path.
Also, the optical isolator has the property of absorbing incident light with wavelengths of 1 micron and less. This is because a YIG (Yttrium-Iron-Garnet) crystal, which is very often used in the optical isolator as a magneto-optical crystal, absorbs light with wavelengths of 1 micron and less.
Accordingly, by arranging the optical isolator 103 in the illustrated position, it is possible to absorb the pump light having a wavelength of 0.98 microns. As a result, only the optical signal from which the pump light is removed is input to individual devices succeeding the EDF 102, making it possible to avoid the situation where the optical signal containing the pump light enters the PD 108, for example, which causes errors in the monitored signal level.
As conventional techniques, there has also been proposed an optical amplifier using an optical isolator module which has a single optical isolator arranged between two collimating lenses and which functions as if it has two optical isolators (e.g., Unexamined Japanese Patent Publication No. H09-54285 (paragraph nos. [0015] to [0022], FIG. 2)).
An optical amplifier is constituted by various devices as mentioned above, and among them, an optical isolator in particular is an optical device which plays an important role in enhancing the quality of optical amplification control. However, an optical isolator is one of the most expensive optical passive devices and entails an increase in cost (in the optical amplifier 100, an expensive optical isolator 103 is used to cut off the 0.98-micron pump light). Also, the use of an optical isolator makes it difficult to reduce the size of the device.
In the optical amplifier 100, on the other hand, the optical signal output from the EDF 102 needs to be monitored through the PD 108, in order to perform feedback control on the pump light power. To permit the optical signal monitoring, the BS 105 is used to split the optical signal, but since the BS 105 arranged on the optical signal (main optical signal) transmission path is a lossy medium entailing optical loss, deterioration in the OSNR (Optical Signal/Noise Ratio) is caused. It is therefore desirable that the transmission path through which the main optical signal is propagated should include the smallest possible number of lossy media as optical components.